Bacterial deposits, removed from the subgingival surfaces of teeth having healthy and diseased periodontal tissues, are being assessed by phase-contrast microscopy, fluorescent antibody techniques, and cultural methods. The bacterial complexes associated with disease differ markedly from those associated with health. Phase contrast microscopy reveals different complexes formed by stationary rods, cocci, spirochetes, motile rods, and amoebae. Populations of actinomyces also appear to differ. Some of the motile rods that have been isolated and studied appear to be strains of Cytophaga. Only a few of the highly prevalent spirochital populations have been isolated. Amoebae have been isolated, but maintained with difficulty. As determined by phase contrast microscopy, populations associated with disease can be shifted to those associated with health by antibacterial regimens that include: (1) debridement and decontamination of roots by the clinician, (2) the patient's daily use of certain inorganic salts and hydrogen peroxide to disperse and inactive root surface bacteria, (3) the occasional administration of tetracycline to suppress the microbiota not reliably controlled by the first two measures. Bleeding, suppuration, ulceration, hyperplasia, tooth mobility, halitosis, and resorption of bone abate as the microbiota associated with disease changes to that associated with health.